In general, heavy equipment, such as a forklift truck and a wheel loader, is used for work of raising or lowering a heavy object (or freight) and work of carrying an object to a desired place, and has been used throughout the industry. Briefly describing a forklift truck, the car body of the forklift truck is supported by the front driving wheels and the rear steering wheels and a mast assembly is disposed at the front. The mast assembly is disposed with the lower end rotatable about a driving shaft supporting the front driving wheels and has a mast rail vertically disposed, and a fork carriage that moves up/down along the mast rail and moves left/right is mounted on the mast rail.
FIG. 1 is a hydraulic circuit diagram of a hydraulic system of a forklift truck according to the related art.
A forklift truck according to the related art, as show in FIG. 1, includes first and second pumps 1 and 2, a priority valve 3, a main control valve unit 10, two lift cylinders 4, two tilt cylinders 5, a lift lever 6 with a remote control valve 6a, and a tilt lever 7 with a remote control valve 7a. 
In this configuration, the first and second pumps 1 and 2 are operated by receiving power from an engine, the first pump 1 is connected with the port at a side of the main control valve unit 10 and the second pump is connected with the port at the other side of the main control valve unit 10 through the priority valve 3.
The priority valve 3 is operated by a pressure difference between pressure receiving portions at both sides and serves to guide pressure oil from the second pump 2 to a steering unit or the main control valve unit 10 and the steering unit.
The main control valve unit 10 includes two lift cylinder operation units 11 and 12 and one tilt cylinder operation unit 13. The two lift cylinder operation units 11 and 12 control the two lift cylinders 4 that move the fork carriage (not shown) up/down along the mast rail of the mast assembly and the tilt cylinder operation unit 13 controls the two tilt cylinders 5 that tilt the mast assembly forward/rearward with respect to the forklift truck.
The lift cylinders 4 are controlled by the lift cylinder operation units 11 and 12 of the main control valve unit 10 by the lift lever 6 in a forklift truck cabin and the tilt cylinders 5 are controlled through the tilt cylinder operation unit 13 of the main control valve unit 10 by the tilt lever 7 in the cabin.
However, when the forklift truck according to the related art lifts a working machine (or fork carriage) for raising or lowering a heavy object (or freight), the two lift cylinder operation units 11 and 12 of the main control valve unit 10 are opened by pilot pressure of the remote control valve 6a controlled by the lift lever 6 regardless of the RPM of the engine and the entire amount of oil supplied from both the first and second pumps 1 and 2 is supplied to the two lift cylinders 4 through the two lift cylinder operation units 11 and 12.
That is, in the forklift truck according to the related art, since the amount of oil supplied from both the first and second pumps 1 and 2 is supplied to the two lift cylinders 4 through the two lift cylinder operation units 11 and 12 even if the RPM of the engine is low, the load exerted in both the first and second pumps 1 and 2 through the two lift cylinders 11 and 12 may be applied to the engine.
When the engine acceleration pedal is pressed down and the fork carriage (not shown) is raised, with the load applied to the engine through the two pumps at low RPM of the engine, it takes long time for the RPM of the engine to reach the maximum level in the forklift truck, such that the work performance is deteriorated and the worker feels inconvenience.
On the other hand, when the engine acceleration pedal is not pressed down at low RPM of the engine and the fork carriage (not shown) with a large load mounted is raised, the operation of the engine is likely to stop when the load exerted in both the first and second pumps 1 and 2 is larger than the corresponding output torque of the engine.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.